In an effort to prevent abuse, Google will temporarily disable your Gmail account if you send messages to the more than 500 recipients or if you send a large number of undeliverable messages. i.e.
The Gmail sending limit for any single email is 500.
- The Gmail sending limit for an 24 hour period is 500.
Given Google's sending limits, Flashissue recommends keeping the number of recipients you send your email newsletter to under 250 people in any given 24 hour period.
This will give ample room for you to send your email newsletter and leave capacity to send your everyday emails through Gmail as well.
Here's a blog post that goes into more detail on this subject:
Gmail sending limits for free Gmail users.